Gallium nitride based diodes with low forward voltage and low reverse current operation

ABSTRACT

New Group III based diodes are disclosed having a low on state voltage (V f ) and structures to keep reverse current (I rev ) relatively low. One embodiment of the invention is Schottky barrier diode made from the GaN material system in which the Fermi level (or surface potential) of is not pinned. The barrier potential at the metal-to-semiconductor junction varies depending on the type of metal used and using particular metals lowers the diode&#39;s Schottky barrier potential and results in a V f  in the range of 0.1-0.3V. In another embodiment a trench structure is formed on the Schottky diodes semiconductor material to reduce reverse leakage current. and comprises a number of parallel, equally spaced trenches with mesa regions between adjacent trenches. A third embodiment of the invention provides a GaN tunnel diode with a low Vf resulting from the tunneling of electrons through the barrier potential, instead of over it. This embodiment can also have a trench structure to reduce reverse leakage current.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to diodes, and more particularly togallium nitride (GaN) based diodes exhibiting improved forward voltageand reverse leakage current characteristics.

[0003] 2. Description of the Related Art

[0004] Diode rectifiers are one of the most widely used devices for lowvoltage switching, power supplies, power converters and relatedapplications. For efficient operation it is desirable for diodes to havelow on-state voltage (0.1-0.2V or lower), low reverse leakage current,high voltage blocking capability (20-30V), and high switching speed.

[0005] The most common diodes are pn-junction diodes made from silicon(Si) with impurity elements introduced to modify, in a controlledmanner, the diode's operating characteristics. Diodes can also be formedfrom other semiconductor materials such as Gallium Arsenide (GaAs) andsilicon carbide (SiC). One disadvantage of junction diodes is thatduring forward conduction the power loss in the diode can becomeexcessive for large current flow.

[0006] Schottky barrier diodes are a special form of diode rectifierthat consist of a rectifying metal-to-semiconductor barrier area insteadof a pn junction. When the metal contacts the semiconductor a barrierregion is developed at the junction between the two. When properlyfabricated the barrier region will minimize charge storage effects andimprove the diode switching by shortening the turn-off time. [L. P.Hunter, Physics of Semiconductor Materials, Devices, and Circuits,Semiconductor Devices, Page 1-10 (1970)] Common Schottky diodes have alower turn-on voltage (approximately 0.5V) than pn-junction diodes andare more desirable in applications where the energy losses in the diodescan have a significant system impact (such as output rectifiers inswitching power supplies).

[0007] One way to reduce the on-state voltage below 0.5V in conventionalSchottky diodes is to reduce their surface barrier potential. This,however, results in a trade-off of increased reverse leakage current. Inaddition, the reduced barrier can degrade high temperature operation andresult in soft breakdown characteristics under reverse bias operation.

[0008] Also, Schottky diodes are commonly made of GaAs and onedisadvantage of this material is that the Fermi level (or surfacepotential) is fixed or pinned at approximately 0.7 volts. As a result,the on-state forward voltage (V_(f)) is fixed. Regardless of the type ofmetal used to contact the semiconductor, the surface potential cannot belowered to lower the V_(f).

[0009] More recently, silicon based Schottky rectifier diodes have beendeveloped with a somewhat lower V_(f). [IXYS Corporation, Si Based PowerSchottky Rectifier, Part Number DSS 20-0015B; International Rectifier,Si Based Shottky Rectifier, Part Number 11DQ09]. The Shottky barriersurface potential of these devices is approximately 0.4V with the lowerlimit of V_(f) being approximately 0.3-0.4 volts. For practical purposesthe lowest achievable Shottky barrier potential is around 0.4 volts withregular metalization using titanium. This results in a V_(f) ofapproximately 0.25V with a current density of 100 A/cm².

[0010] Other hybrid structures have been reported with a V_(f) ofapproximately 0.25V (with a barrier height of 0.58V) with operatingcurrent density of 100 A/cm². [M. Mehrotra, B. J. Baliga, “The TrenchMOS Barrier Shottky (TMBS) Rectifier”, International Electron DeviceMeeting, 1993]. One such design is the junction barrier controlledSchottky rectifier having a pn-junction used to tailor the electricfields to minimize reverse leakage. Another device is the trench MOSbarrier rectifier in which a trench and a MOS barrier action are used totailor the electrical field profiles. One disadvantage of this device isthe introduction of a capacitance by the pn-junction. Also, pn-junctionsare somewhat difficult to fabricate in Group III nitride based devices.

[0011] The Gallium nitride (GaN) material system has been used inopto-electronic devices such as high efficiency blue and green LEDs andlasers, and electronic devices such as high power microwave transistors.GaN has a 3.4 eV wide direct bandgap, high electron velocity (2×10⁷cm/s), high breakdown fields (2×10⁶ V/cm) and the availability ofheterostructures.

SUMMARY OF THE INVENTION

[0012] The present invention provides new Group III nitride based diodeshaving a low V_(f). Embodiments of the new diode also include structuresto keep reverse current (I_(rev)) relatively low.

[0013] The new diode is preferably formed of the GaN material system,and unlike conventional diodes made from materials such as GaAs, theFermi level (or surface potential) of GaN is not pinned at its surfacestates. In GaN Schottky diodes the barrier height at themetal-to-semiconductor junction varies depending on the type of metalused. Using particular metals will lower the diode's Schottky barrierheight and result in a V_(f) in the range of 0.1-0.3V.

[0014] The new GaN Schottky diode generally includes an n+ GaN layer ona substrate, and an n− GaN layer on the n+ GaN layer opposite thesubstrate. Ohmic metal contacts are included on the n+ GaN layer,isolated from the n− GaN layer, and a Schottky metal layer is includedon the n− GaN layer. The signal to be rectified is applied to the diodeacross the Schottky metal and ohmic metal contacts. When the Schottkymetal is deposited on the n− GaN layer, a barrier potential forms at thesurface of said n− GaN between the two. The Schottky metal layer has awork function, which determines the height of the barrier potential.

[0015] Using a metal that reduces the Schottky barrier potential resultsin a low V_(f), but can also result in an undesirable increase inI_(rev). A second embodiment of the present invention reduces I_(rev) byincluding a trench structure on the diode's surface. This structureprevents an increase in the electric field when the new diode is underreverse bias. As a result, the Schottky barrier potential is lowered,which helps reduce I_(rev).

[0016] The trench structure is preferably formed on the n− GaN layer,and comprises a number of parallel, equally spaced trenches with mesaregions between adjacent trenches. Each trench has an insulating layeron its sidewalls and bottom surface. A continuous Schottky metal layeris on the trench structure, covering the insulating layer and the mesasbetween the trenches. Alternatively, the sidewalls and bottom surface ofeach trench can be covered with metal instead of an insulator, with themetal electrically isolated from the Schottky metal. The mesa regionshave a doping concentration and width chosen to produce the desiredredistribution of electrical field under the metal-semiconductorcontact.

[0017] A third embodiment of the invention provides a GaN tunnel diodewith a low V_(f) resulting from the tunneling of electrons through thebarrier potential, instead of over it. This embodiment has a substratewith an n+ GaN layer sandwiched between the substrate and an n− GaNlayer. An AlGaN barrier layer is included on the n− GaN layer oppositethe n+ GaN layer. An Ohmic contact is included on the n+ GaN layer and atop contact is included on the AlGaN layer. The signal to be rectifiedis applied across the Ohmic and top contacts.

[0018] The barrier layer design maximizes the forward tunnelingprobability while the different thickness and Al mole fraction of thebarrier layer result in different forward and reverse operatingcharacteristics. At a particular thickness and Al mole fraction, thediode has a low V_(f) and low I_(rev). Using a thicker barrier layerand/or increasing the Al mole concentration decreases V_(f) andincreases I_(rev). As the thickness or mole fraction is increasedfurther, the new diode will assume ohmic operating characteristics, orbecome a conventional Schottky diode.

[0019] These and other further features and advantages of the inventionwould be apparent to those skilled in the art from the followingdetailed description, taking together with the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a sectional view of a GaN Schottky diode embodiment ofthe invention;

[0021]FIG. 2 is a diagram showing the work function of common metalsverses their atomic number;

[0022]FIG. 3 is a band diagram for the diode shown in FIG. 1;

[0023]FIG. 4 is a sectional view of another embodiment of the GaNSchotty diode of FIG. 1, having a trench structure to reduce reversecurrent leakage;

[0024]FIG. 5 is a sectional view of a tunnel diode embodiment of theinvention;

[0025]FIG. 6 is a band diagram for the tunnel diode of FIG. 5 having abarrier layer with a thickness of 22 Å and 30% Al mole fraction;

[0026]FIG. 7 is a diagram showing the voltage/current characteristics ofthe new tunnel diode having the band diagram of FIG. 6;

[0027]FIG. 8 is a band diagram for the tunnel diode of FIG. 5 having abarrier layer with a thickness of 30 Å and 30% Al mole fraction;

[0028]FIG. 9 is a diagram showing the voltage/current characteristics ofthe new tunnel diode having the band diagram of FIG. 8;

[0029]FIG. 10 is a band diagram for the tunnel diode of FIG. 5 having abarrier layer with a thickness of 38 Å and 30% Al mole fraction;

[0030]FIG. 11 is a diagram showing the voltage/current characteristicsof the new tunnel diode having the band diagram of FIG. 10; and

[0031]FIG. 12 is a sectional view of a tunnel diode embodiment of theinvention having a trench structure to reduce reverse current leakage.

DETAILED DESCRIPTION OF THE INVENTION

[0032]FIG. 1 shows a Schottky diode 10 constructed in accordance withthe present invention having a reduced metal-to-semiconductor barrierpotential. The new diode is formed of the Group III nitride basedmaterial system or other material systems where the Fermi level is notpinned at its surface states. Group III nitrides refer to thosesemiconductor compounds formed between nitrogen and the elements inGroup III of the periodic table, usually aluminum (Al), gallium (Ga),and indium (In). The term also refers to ternary and tertiary compoundssuch as AlGaN and AlInGaN. The preferred materials for the new diode areGaN and AlGaN.

[0033] The new diode 10 comprises a substrate 11 that can be eithersapphire (Al₂O₃), silicon (Si) or silicon carbide (SiC), with thepreferred substrate being a 4H polytype of silicon carbide. Othersilicon carbide polytypes can also be used including 3C, 6H and 15Rpolytypes. An Al_(x)Ga¹⁻N buffer layer 12 (where x in between 0 and 1)is included on the substrate 11 and provides an appropriate crystalstructure transition between the silicon carbide substrate and theremainder of the diode 10.

[0034] Silicon carbide has a much closer crystal lattice match to GroupIII nitrides than sapphire and results in Group III nitride films ofhigher quality. Silicon carbide also has a very high thermalconductivity so that the total output power of Group III nitride deviceson silicon carbide is not limited by the thermal dissipation of thesubstrate (as is the case with some devices formed on sapphire). Also,the availability of silicon carbide substrates provides the capacity fordevice isolation and reduced parasitic capacitance that make commercialdevices possible. SiC substrates are available from Cree Research, Inc.,of Durham, N.C. and methods for producing them are set forth in thescientific literature as well as in a U.S. Pat. Nos. Re. 34,861;4,946,547; and 5,200,022.

[0035] The new diode 10 has an n+ GaN layer 12 on a substrate 11 and ann− layer of GaN 13 on the n+ GaN layer 12, opposite the substrate 11.The n+ layer 12 is highly doped with impurities to a concentration of atleast 10¹⁸ per centimeter cubed (cm³), with the preferable concentrationbeing 5 to 10 times this amount. The n− layer 13 has a lower dopingconcentration but is still n− type and it preferably has an impurityconcentration in the range of 5×10¹⁴ to 5×10¹⁷ per cm³. The n-layer 13is preferably 0.5-1 micron thick and the n+ layer 12 is 0.1 to 1.5microns thick, although other thicknesses will also work.

[0036] Portions of the n− GaN layer 13 are etched down to the n+ layerand ohmic metal contacts 14 a and 14 b are included on the n+ GaN layerin the etched areas so that they are electrically isolated from the n−GaN layer 13. In an alternative embodiment, one or more ohmic contactscan be included on the surface of the substrate that is not covered bythe n+ GaN layer 12. This embodiment is particularly applicable tosubstrates that are n-type. A Schottky metal layer 16 is included on then− GaN layer 13, opposite the n+ GaN layer 12.

[0037] The work function of a metal is the energy needed to take anelectron out of the metal in a vacuum and the Fermi level of a materialis the energy level at which there is a 50% probability of finding acharged carrier. A semiconductor's electron affinity is the differencebetween its vacuum energy level and the conduction band energy level.

[0038] As described above, the surface Fermi level of GaN is unpinnedand as a result, Schottky metals with different work functions result indifferent barrier potentials. The barrier potential is approximated bythe equation: $\text{Barrier~~Height} = \begin{matrix}{\text{work~~function} -} \\{\text{the~~}{{{semiconductor}'}s\quad {electron}\quad {affinity}}}\end{matrix}$

[0039]FIG. 2 is a graph 20 showing the metal work function 21 forvarious metal surfaces in a vacuum, verses the particular metal's atomicnumber 22. The metal should be chosen to provide a low Schottky barrierpotential and low V_(f), but high enough so that the reverse currentremains low. For example, if a metal were chosen having a work functionequal to the semiconductor's electron affinity, the barrier potentialapproaches zero. This results in a V_(f) that approaches zero and alsoincreases the diode's reverse current such that the diode becomes ohmicin nature and provides no rectification.

[0040] Many different metals can be used to achieve a low barrierheight, with the preferred metals including Ti(4.6 work function) 23,Cr(4.7) 24, Nb(4.3) 25, Sn(4.4) 26, W(4.6) 27 and Ta (4.3) 28. Cr 24results in an acceptable barrier potential and is easy to deposit byconventional methods.

[0041]FIG. 3 shows a typical band diagram 30 for the new Schottkybarrier diode taken on a vertical line through the diode. It shows theenergy levels of Schottky metal 31, the GaN semiconductor layers 32, andthe Shottky barrier potential 33.

[0042] Prior to contact of the GaN semiconductor material by theSchottky metal, the Fermi energy levels of the two are not the same.Once the contact is made and the two materials become a singlethermodynamic system, a single Fermi level for the system results. Thisis accomplished by the flow of electrons from the semiconductormaterial, which has a higher Fermi level, to the Schottky metal, whichhas a lower Fermi level. The electrons of the semiconductor lower theirenergy by flowing into the metal. This leaves the ionized donor levelsof the semiconductor somewhat in excess of the number of its freeelectrons and the semiconductor will have a net positive charge.Electrons that have flowed from the semiconductor into the metal causethe metal have a negative electrostatic charge. The energy levels of thesemiconductor are accordingly depressed, and those of the metal areraised. The presence of this surface charge of electrons and thepresence of unneutralized charge ionized donor levels of thesemiconductor create the dipole layer which forms the barrier potential.

[0043] In operation, the signal to be rectified by the new Schottkydiode 10 is applied across the Schottky metal 14 and the ohmic contacts14 a and 14 b. The rectification of the signal results from the presenceof the barrier potential at the surface of the n− GaN layer 13, whichinhibits the flow of charged particles within the semiconductor. Whenthe Schottky metal 16 is positive with respect to the semiconductor(forward bias), the energy at the semiconductor side of the barrier israised. A larger number of free electrons on the conduction band arethen able to flow into the metal. The higher the semiconductor side israised, the more electrons there are at an energy above the top of thebarrier, until finally, with large bias voltages the entire distributionof free electrons in the semiconductor is able to surmount the barrier.The voltage verses current characteristics become Ohmic in nature. Thelower the barrier the lower the V_(f) necessary to surmount the barrier.

[0044] However, as discussed above, lowering the barrier level can alsoincrease the reverse leakage current. When the semiconductor is madepositive with respect to the metal (reverse bias), the semiconductorside of the barrier is lowered relative to the metal side so that theelectrons are free to flow over the top of the barrier to thesemiconductor unopposed. The number of electrons present in the metalabove the top of the barrier is generally very small compared to thetotal number of electrons in the semiconductor. The result is a very lowcurrent characteristic. When the voltage is large enough to cut-off allflow of electrons, the current will saturate. The lower the barrierpotential, the smaller reverse biases needed for the current tosaturate.

[0045]FIG. 4 shows another embodiment of the new GaN Schottky diode 40that addresses the problem of increased reverse current with decreasedbarrier height. The diode 40 is similar to the above embodiment having asimilar substrate 41, n+ GaN layer 42, and Ohmic metal contacts 43 a and43 b, that can alternatively be included on the surface of thesubstrate. It also has an n− GaN layer 44, but instead of this layerbeing planar, it has a two dimensional trench structure 45 that includestrenches 46 in the n− GaN layer. The preferred trench structure 45includes trenches 46 that are parallel and equally spaced with mesaregions 49 remaining between adjacent trenches. Each trench 46 has aninsulating layer 47 covering its sidewalls 46 a and bottom surface 46 b.Many different insulating materials can be used with the preferredmaterial being silicon nitride (SiN). A Schottky metal layer 48 isincluded over the entire trench structure 45, sandwiching the insulatinglayer between the Schottky metal and the trench sidewalls and bottomsurface, and covering the mesa regions 49. The mesa regions provide thedirect contact area between the Schottky metal and the n− GaN layer 44.Alternatively, each trench can be covered by a metal instead of aninsulator. In this embodiment, the Schottky metal should be insulatedand/or separated from the trench metal.

[0046] The mesa region 49 has a doping concentration and width chosen toproduce a redistribution of electrical field under the mesa'smetal-semiconductor junction. This results in the peak of the diodeselectrical field being pushed away from the Schottky barrier and reducedin magnitude. This reduces the barrier lowering with increased reversebias voltage, which helps prevent reverse leakage current fromincreasing rapidly.

[0047] This redistribution occurs due to the coupling of the charge inthe mesa 49 with the Schottky metal 48 on the top surface and with themetal on the trench sidewalls 46 a and bottom surface 46 b. Thedepletion then extends from both the top surface (as in a conventionalSchottky rectifier) and the trench sidewalls 46 a, depleting theconduction area from the sidewalls. The sidewall depletion reduces theelectrical field under the Schottky metal layer 48 and can also bethought of as “pinching off” the reverse leakage current. The trenchstructure 45 keeps the reverse leakage current relatively low, even witha low barrier potentials and a low V_(f).

[0048] The preferred trench structure 45 has trenches 46 that are one totwo times the width of the Schottky barrier area. Accordingly, if thebarrier area is 0.7 to 1.0 microns, the trench width could be in therange of 0.7 to 2 microns.

[0049] The above diodes 10 and 40 are fabricated using known techniques.Their n+ and n− GaN layers are deposited on the substrate by knowndeposition techniques including but not limited to metal-organicchemical vapor deposition (MOCVD). For diode 10, the n− GaN layer 13 isetched to the n+ GaN layer 12 by known etching techniques such aschemical, reactive ion etching (RIE), or ion mill etching. The Schottkyand Ohmic metal layers 14, 14 b and 16 are formed on the diode 10 bystandard metallization techniques.

[0050] For diode 40, after the n+ and n− layers 42 and 44 are depositedon the substrate, the n− GaN layer 44 is etched by chemical or ion milletching to form the trenches 46. The n− GaN layer 44 is further etchedto the n+ GaN layer 42 for the ohmic metal 43 a and 43 b. The SiNinsulation layer 47 is then deposited over the entire trench structure45 and the SiN layer is etched off the mesas 49. As a final step, acontinuous Schottky metal layer 48 is formed by standard metalizationtechniques over the trench structure 45, covering the insulation layers47 and the exposed trench mesas 49. The ohmic metal is also formed onthe n+ GaN layer 42 by standard metalization techniques. In theembodiments of the trench diode where the trenches are covered by ametal, the metal can also be deposited by standard metalizationtechniques.

[0051] Tunnel Diode

[0052]FIG. 5 shows another embodiment 50 of the new diode wherein V_(f)is low as a result of electron tunneling through the barrier regionunder forward bias. By tunneling through the barrier electrons do notneed to cross the barrier by conventional thermionic emission over thebarrier.

[0053] Like the embodiments in FIGS. 1 and 4, the new tunnel diode 50 isformed from the Group III nitride based material system and ispreferably formed of GaN, AlGaN or InGaN, however other material systemswill also work. Combinations of polar and non-polar materials can beused including polar on polar and polar on non-polar materials. Someexamples of these materials include complex polar oxides such asstrontium titanate, lithium niobate, lead zirconium titanate, andnon-complex/binary oxides such as zinc oxide. The materials can be usedon silicon or any silicon/dielectric stack as long as tunneling currentsare allowed.

[0054] The diode 50 has a substrate 51 comprised of either sapphire,silicon carbide (SiC) or silicon Si, with SiC being the preferredsubstrate material for the reasons outlined above. The substrate has ann+ GaN layer 52 on it, with an n− GaN layer 53 on the n+ GaN layer 52opposite the substrate 51. An AlGaN barrier layer 54 is included on then− GaN layer opposite the n+ GaN template layer 52. At the edges of thediode 50, the barrier layer 54 and n− GaN layer 53 are etched down tothe n+ GaN layer 52 and ohmic metal contacts 55 a and 55 b are includedon the layer 52 in the etched areas. As with the above structures, theohmic contacts can also be included on the surface of the substrate. Ametal contact layer 56 is included on the AlGaN barrier layer 54,opposite the n− GaN layer 53. The signal to be rectified is appliedacross the ohmic contacts 55 a and 55 b and top metal contact 56.

[0055] The AlGaN barrier layer 54 serves as a tunnel barrier. Tunnelingacross barriers is a quantum mechanical phenomenon and both thethickness and the Al mole fraction of the layer 54 can be varied tomaximize the forward tunneling probability. The AlGaN—GaN materialsystem a has built in piezoelectric stress, which results inpiezoelectric dipoles. Generally both the piezoelectric stress and theinduced charge increases with the barrier layer thickness. In theforward bias, the electrons from the piezoelectric charge enhancetunneling since they are available for conduction so that the number ofstates from which tunneling can occur is increased. Accordingly the newtunnel diode can be made of other polar material exhibiting this type ofpiezoelectric charge.

[0056] However, under a reverse bias the piezoelectric charge alsoallows an increase in the reverse leakage current. The thicker thebarrier layer or increased Al mole fraction, results in a lower V_(f)but also results in an increased I_(rev). Accordingly, there is anoptimum barrier layer thickness for a particular Al mole fraction of thebarrier layer to achieve operating characteristics of low V_(f) andrelatively low I_(rev)

[0057] FIGS. 6-11 illustrate the new diode's rectificationcharacteristics for three different thicknesses of an AlGaN barrierlayer with 30% Al. For each thickness there is a band energy diagram anda corresponding voltage vs. current graph

[0058]FIG. 6 shows the band diagram 60 for the tunnel diode 50 having 22Å thick barrier layer 54. It shows a typical barrier potential 61 at thejunction between the barrier layer 63 and the n− GaN semiconductor layer62. The top contact metal 64 is on the barrier layer 63, opposite thesemiconductor layer. FIG. 7 shows a graph 70 plotting the correspondingcurrent vs. voltage characteristics of the diode in FIG. 6. It has aV_(f 71 of approximately) 0.1V and low reverse current (I_(rev)) 72.

[0059]FIG. 8 shows a band diagram 80 for the same tunnel diode with a 30Å thick barrier layer. The increase in the barrier layer thicknessincreases the barrier region's piezoelectric charge, thereby enhancingtunneling across the barrier. This flattens the barrier potential 81 atthe junction between the barrier layer 82 and the n− GaN layer 83.Charges do not need to overcome the barrier when a forward bias isapplied, greatly reducing the diode's V_(f) . However, the flattenedbarrier also allows for increase reverse leakage current (I_(rev)). FIG.9 is a graph 90 showing the V_(f 91 that is lower than the V) _(f) inFIG. 7. Also, I_(rev 92 is increased compared to I) _(rev) in FIG. 7.

[0060]FIG. 10 shows a band diagram 100 for the same tunnel diode with a38 Å thick barrier layer. Again, the increase in the barrier layerthickness increases the piezoelectric charge. At this thickness, thebarrier potential 101 between the barrier layer 102 and n− GaN layertails down near the junction between the barrier layer and n− GaN layer,which results in there being no barrier to charges in both forward andreverse bias. FIG. 11 shows a graph 110 of the corresponding current vs.voltage characteristics. The diode 100 experiences immediate forward andreverse current in response to forward and reverse bias such that thediode becomes ohmic in nature.

[0061] In the case where the mole concentration of aluminum in thebarrier layer is different, the thicknesses of the layers would bedifferent to achieve the characteristics shown in FIGS. 6 through 11.

[0062]FIG. 12 shows the new tunneling diode 120 with a trench structure121 to reduce reverse current. Like the Schottky diode 40 above, thetrench structure includes a number of parallel, equally spaced trenches122, but in this diode, they are etched through the AlGaN barrier layer123 and the n− GaN layer 124, to the n+ GaN layer 125 (AP GaN Template).There are mesa regions 126 between adjacent trenches 122. The trenchsidewalls and bottom surface have an insulation layer 127 with the topSchottky metal layer 128 covering the entire trench structure 121. Thetrench structure functions in the same way as the embodiment above,reducing the reverse current. This is useful for the tunnel diodeshaving barrier layers of a thickness that results in immediate forwardcurrent in response to forward voltage. By using trench structures, thediode could also have improved reverse current leakage. Also like above,the trench sidewalls and bottom surface can be covered by a metal aslong as it is isolated from the Schottky metal layer 128.

[0063] Although the present invention has been described in considerabledetail with reference to certain preferred configurations thereof, otherversions are possible. Therefore, the spirit and scope of the appendedclaims should not be limited to the preferred versions described in thespecification.

We claim:
 1. A group III nitride based diode, comprising: an n+ dopedGaN layer; an n− doped GaN layer on said n+ GaN layer; a Schottky metallayer on said n− doped GaN layer having a work function, said n− GaNlayer forming a junction with said Schottky metal, said junction havinga barrier potential energy level that is dependent upon the workfunction of said Schottky metal.
 2. The diode of claim 1, wherein saidbarrier potential varies directly with said Schottky metal workfunction.
 3. The diode of claim 1, wherein said n− doped GaN layer hasan electron affinity, said barrier potential being generally equal tosaid Schottky metal work function minus said electron affinity.
 4. Thediode of claim 1, further comprising a substrate adjacent to said n+ GaNlayer, opposite said n− doped GaN layer.
 5. The diode of claim 4,wherein said substrate is sapphire (Al₂O₃), silicon carbide (SiC) orsilicon (Si)
 6. The diode of claim 1, wherein said Schottky metal is oneof the metals from the group comprising Ti, Cr, Nb, Sn, W, Ta and Ge. 7.The diode of claim 1, wherein said n+ doped GaN layer is doped withimpurities to a concentration of at least 10¹⁸ per centimeter cubed(cm³)
 8. The diode of claim 1, wherein the n− doped GaN layer is dopedwith impurities to a concentration in the range of 5×10¹⁴ to 5×10¹⁷ percm³.
 9. The diode of claim 1, further comprising a trench structure insaid n− doped GaN layer, said diode experiencing a reverse leakagecurrent under reverse bias, said trench structure reducing said reverseleakage current.
 10. The diode of claim 9, wherein said trench structurecomprises a plurality of trenches with mesa regions between adjacenttrenches, said trenches having sidewalls and a bottom surface coated byan insulating material, said Schottky metal layer covering said trenchesand mesa regions, said insulating material sandwiched between saidSchottky metal layer and said sidewalls and bottom surfaces.
 11. Thediode of claim 10, wherein said plurality of trenches are parallel andequally spaced.
 12. The diode of claim 10, wherein said insulatingmaterial is SiN.
 13. The diode of claim 10, wherein said insulatingmaterial is replaced by a metal with a high work function.
 14. The diodeof claim 1, further comprising an ohmic contact on said n+ GaN layer, asignal applied to said device across said ohmic contact and saidSchottky metal layer.
 15. A diode, comprising: a layer of highly dopedsemiconductor material having an unpinned surface potential; a layer oflower doped semiconductor material adjacent to the highly dopedsemiconductor material; and a Schottky metal layer on said lower dopedsemiconductor material, said lower doped semiconductor material forminga junction with said Schottky metal having a barrier potential energylevel that is dependent upon the type of Schottky metal.
 16. The diodeof claim 15, wherein said doped layers are doped n type.
 17. The diodeof claim 15, wherein said semiconductor material is a Group III nitride.18. The diode of claim 15, wherein said highly doped semiconductor is n+doped GaN layer and said lower doped semiconductor is n− doped GaNlayer.
 19. The diode of claim 15, wherein said Schottky metal contacthas a work function, said barrier potential having an energy level thatvaries directly with the work function of said Schottky metal.
 20. Thediode of claim 15, further comprising a substrate adjacent to said n+doped GaN layer, opposite said n− doped GaN layer.
 21. The diode ofclaim 20, wherein said substrate is sapphire (Al₂O₃), silicon carbide(SiC) or silicon (Si)
 22. The diode of claim 15, wherein said Schottkymetal is one of the metals in the group comprising Ti, Cr, Nb, Sn, W, Geand Ta.
 23. The diode of claim 18, wherein said n+ doped GaN layer isdoped with impurities to a concentration of at least 10¹⁸ per centimetercubed (cm³).
 24. The diode of claim 18, wherein the n− doped GaN layeris doped with impurities to a concentration in the range of 5×10¹⁴ to5×10¹⁷ per cm³.
 25. The diode of claim 15, further comprising a trenchstructure on the surface of said lower doped semiconductor material,said diode experiencing a reverse leakage current under reverse bias,said trench structure reducing the amount of reverse leakage current.26. The diode of claim 25, wherein said trench structure comprises aplurality of trenches with mesa regions between adjacent trenches, eachof said trenches having sidewalls and a bottom surface coated by aninsulating material, said Schottky metal layer covering said trenchesand mesa regions, said insulating material sandwiched between saidSchottky metal layer and said sidewalls and bottom surfaces.
 27. Thediode of claim 26, wherein said insulating material is replaced by ametal with a high work function.
 28. The diode of claim 15, furthercomprising an ohmic contact on said higher doped semiconductor material.29. A tunneling diode comprising: an n+ doped layer; an n− doped layeradjacent to said n+ doped layer; a barrier layer adjacent to said n−doped layer, opposite said n+ layer; and a metal layer on said barrierlayer, opposite said n-doped layer, said n− doped layer forming ajunction with said barrier layer that has a barrier potential whichcauses said diode's on state voltage to be low as a result of electrontunneling through the barrier potential under forward bias.
 30. Thediode of claim 29, wherein said barrier layer has piezoelectric dipolesthat lower the diode's on state voltage by enhancing electron tunneling.31. The diode of claim 29, wherein the number of piezoelectric dipolesincreases as the thickness of said barrier layer increases, while stillallowing tunneling currents.
 32. The diode of claim 29, furthercomprising a substrate adjacent to said n+ doped layer opposite said n−doped layer, said substrate comprising sapphire, silicon carbide orsilicon.
 33. The diode of claim 29, wherein said n+ doped layer, n−doped layer and barrier layer comprise polar materials.
 34. The diode ofclaim 29, wherein said n+ doped layer, n− doped layer and barrier layerare from the Group III nitride material system.
 35. The diode of claim29, wherein said n+ doped layer is GaN, said n− doped layer is GaN, andsaid barrier layer is AlGaN.
 36. The diode of claim 29, wherein said n+doped layer, n-doped layer and barrier layer are formed from polar ornon-polar materials, or combinations thereof.
 37. The diode of claim 29,wherein said n+ doped layer, n-doped layer and barrier layer are formedfrom complex polar oxides such as strontium titanate, lithium niobate,lead zirconium titanate, or combinations thereof.
 38. The diode of claim29, wherein said n+ doped layer, n-doped layer and barrier layer frombinary polar oxides such as zinc oxide.
 39. The diode of claim 29,further comprising a trench structure in said barrier and n− dopedlayers, said diode experiencing a reverse leakage current under reversebias, said trench structure reducing the amount of said reverse leakagecurrent.
 40. The diode of claim 29, wherein said trench structurecomprises a plurality of trenches in said barrier and said n− layershaving mesa regions between adjacent trenches, each of said trencheshaving sidewalls and a bottom surface coated by an insulating material,said Schottky metal layer covering said trenches and mesa regions, saidinsulating material sandwiched between said Schottky metal layer andsaid sidewalls and bottom surfaces.
 41. The diode of claim 40, whereinsaid insulating material is replaced by a metal with a high workfunction.
 42. The diode of claim 29, further comprising an ohmic contacton said n+ doped layer.
 43. A Schottky diode, comprising: asemiconductor material having an unpinned surface potential; and aSchottky metal having a work function and forming a junction with saidsemiconductor material that has a barrier potential, the height of saidbarrier potential depending upon said work function.
 44. The diode ofclaim 43, wherein said semiconductor material is Group III nitridebased.
 45. The diode of claim 43, wherein said semiconductor layercomprises adjacent n− doped GaN and n+ doped GaN layers.
 46. The diodeof claim 45, further comprising an ohmic contact on said n+ doped GaNlayer, with said Schottky metal contacting said n− GaN layer.
 47. Thediode of claim 43, wherein the height of said barrier potential variespositively with the work function of said Schottky metal.
 48. The diodeof claim 45, further comprising a substrate made of sapphire (Al₂O₃),silicon carbide (SiC) or silicon (Si), adjacent to the said n+ GaNlayer, opposite said n− GaN layer.
 49. The diode of claim 43, whereinsaid Schottky metal is one of the metals in the group comprising Ti, Cr,Nb, Sn, W, Ta, Ge and other metals with similar work functions.
 50. Thediode of claim 43, further comprising a trench structure in saidsemiconductor material, said diode experiencing a reverse leakagecurrent under reverse bias, said trench structure reducing said reverseleakage current.
 51. The diode of claim 43, wherein said trenchstructure comprises a plurality of trenches with mesa regions betweenadjacent trenches, said trenches having sidewalls and a bottom surfacecoated by an insulating material, said Schottky metal layer coveringsaid trenches and mesa regions, said insulating material sandwichedbetween said Schottky metal layer and said sidewalls and bottomsurfaces.